Synthetic paper and process for producing same



Feb. 14, 1961 E. D] GIULIO ETAL 2,971,858

ER AND PROCESS FOR PRODUCING SAME SYNTHETIC PAP Filed June 4, 1957 2 Sheets-Sheet 1 09 o: o- 02 0: 02 02 ot 1N VENTOR 5/1/20 0/ G/UL /0 PA 0L0 PARR/N/ ATTORNEYS SYNTHETIC PAPER ANDPROCESS FOR PRODUCING SAME Filed June 4, 1957 2 Sheets-Sheet 2 Fig.2

F I 5O I00 IN VENTORS [N20 0/ G/UL/O PAOLO FARR/All BY jfiulam F ATTORNEYS United States Patent 01 SYNTHETIC PAPER AND PROCESS FOR PRODUCING SAME Enzo di Giulio, Ferrara, and Paolo Parrini, Terni, Italy, assignors to Monteca'tini So'cieta Generals per llndusllla Mmeraria e Chimica, Milan, Italy This invention relates to a new and novel substitute for conventional paper, making the same.

Conventional paper comprises a Water-laid sheet of matted or interfelted cellulosic fibers. it is made from cellulosic fibers, rags, waste paper, etc. The cellulosic fibers, if required after liberation from rags or waste paper by digestion in suitable digesters, are suspended in water in the beater of the paper-making machine and beaten to obtain a paper-making stock or pulp which is then deposited on a screen from which the water drains away, leaving the cellulosic fibers in the form of a coherent mat which is dried and may be calender-ed.

It is characteristic of the ellulosic fibers that, being natural fibers, they have comparatively rough, somewhat scaly surfaces, and that during the beating they fibrillate. The fibrillae or frayed ends of the individual fibers interfelt so that the adjacent fibers are held together in the coherent mat condition. Both the naturally somewhat scaly surfaces of the cellulosic fibers, and the fibrillae resulting from the beating, contribute a certain roughness to the surface of the final paper, rendering it receptive to inks and capable of being printed or written on easily and iegibly with the Various inks.

Since cellulose is a highly hydroxylated substance, paper as normally manufactured is hydrophilic and readily Wet by Water. It is well known that paper normally loses practically all of its mechanical strength when it is thoroughly wet. As paper is used principally for packing and printing, the high pick-up and moisture retention capacity thereof, and the low wet strength, limit its usefulness. For example, paper is not satisfactory for use at high humidities or under conditions of exposure to water. Accordingly, for many purposes, it is essential to treat the paper in special ways in an effort to improve its wet strength.

Paper has the additional disadvantage that it is not normally crease-resistant, being readily wrinkled when it is bent and retaining the bending marks. In the wrinkled condition, it is no longer suitable for use. For instance, it is extremely difiicult to print or write legibly on paper after it has been bent and creased, even after it has been attempted to again straighten it out. It is not practical to remove the creases by heating under pressure, due to the low decomposition temperature of cellulose.

Many proposals have been made with a view to im" proving the wet strength and crease resistance of paper. Thus, it has been proposed to produce wet strength pa per by incorporating resin-forming materials with the papermaking fibers in the heater, or by impregnating the water-laid web with the resin-forming components, the resinous properties being subsequently developed in the paper with the aid of heat and a suitable catalyst. Such materials are relatively expensive and increase the cost of the paper. Also, if the necessary conditions of temperature, time, concentration, etc. are not strictly controlled, curing of the resin is likely to be more or less incomplete, resulting in variations in the degree of wet synthetic and to methods for strength in the finished paper. In methods in which the resin-forming components are added to the beater, adequate retention thereof in the sheet is an additional problem. Even when the paper is modified by resins, and is classified as a wet strength paper it ordinarily retains only 40% to 50% of its strength when it becomes thoroughiy wet.

Resin and other treatments that have been proposed for increasing the wet strength of paper have the further drawback that, invariably, such treatments have an em brittling eliect, and do not contribute to the capacity of the paper to be written on or printed.

A satisfactory substitute for conventional paper has not been made, so far, from the synthetic resin fibers which have smooth surfaces and do not fibrillate when suspended in water and beaten. Nor has there been disclosed an acceptable paper substitute or paper-like material made from a synthetic resin and capable of being Written on or printed with ink.

One object of this invention is to provide a paper substitute having a more or less rough surface like that possessed by paper as normally manufactured from cellulosic fibers, and which can be printed or written on with the various inks.

This and other objects are accomplished by the cut invention in accordance very valuable, new paper terials can be made from crystalline polypropylene.

In two papers entitled, respectively, The Crystalline Structure of a New Type of Polypropylene and A New Class of Alpha-Olefin Polymers Having Exceptional Uniformity of Structure, presented at an open hearing of Accademia Nazionale Dei Lincei on December 11, 1954, and published in the Proceedings of the Accademia on January 29, 1955, disclosed new linear, regular head-totail high molecular weight crystallizable polymers of propylene and other alpha-olefins which they had produced by polymerizing the alpha-olefine with the aid of catalysts prepared by reacting a compound of a transition metal of groups IV to Vi of the periodic table, such as titanium tetrachloride, with an organometallic compound of an element of the 2nd and 3rd groups of the periodic table, such as triethyl aluminum. Natta et al. showed that the crude polymerizates they obtained are, initially, mixtures of sterieally differentiated linear, head-to-tail macromolecules having no branches longer than R (the CH group in the case of propylene) which mixtures comprise, mainly, amorphous, atactic macromolecules and crystallizable, isotactic macromolecules which can be separated by fractional dissolution.

X-ray examination of the crystallizable macromolecules show that in the monomeric units forming the macromolecules of the polymer and which contain an asymmetric C-atom, the C-atoms have the same steric configuration for long sections of the main chains, the monomeric units being arranged along a spiral having an identity period of 3 monomeric units.

The macromolecules having substantially this very regular structure were first defined by G. Natta, as isotactic macromolecules, and that term has since been adopted by the art.

Because of their high crystallizability resulting from the regular structure, the polymerizates consisting essentially of the isotactic polymers made up of isotactic macromolecules can be used for the production of fibers, films, and other shaped articles having excellent mechanical properties.

We have now found that the paper substitutes which are the subject of this invention, can be made from the new isotactic polypropylene by spreading a solution of the polymer as a thin layer on a support and evaporating the solvent under conditions as explained hereinbelow.

preswith which it is found that substitutes or paper-like ma- The polypropylenes which may be used (intrinsic viscosity of 0.5-6.0 determined in tetralin at 135 C.) are not soluble in the cold in any solvent. However they can be dissolved at elevated temperature (60 C.- 120 C.) in various solvents including petroleum fractions boiling at 100 C. to 250 C., toluene, xylene, tetrahydro-naphthalene, decahydro-naphthalene, diphenyl, diphenyl-oxide, and chlorobenzene.

However, since the sheet is formed by spreading the solution as a thin layer on a support, care must be exercised in selecting the solvent to choose those which do not yield gels and which give solutions that are readily spreadable at useful concentrations of the polymer.

For instance, Decalin dissolves isotactic polypropylene at the elevated temperature to yield solutions of up toabout 15% concentration. However, the solutions are readily spreadable only at concentrations of the polymer below 5-7%. Solutions in Decalin having a higher concentration than 57% can be spread only with difliculty because those solutions have the characteristics of gels rather than the characteristics of true solutions.

Xylene dissolves the polypropylene in concentrations up to but only those solutions having a concentration of about 3.5% or less can be spread easily.

Toluene is a poor solvent for the isotactic polypropylene even at the elevated temperatures. The solutions have a maximum concentration of only 34-%, and are elastic gels which can hardly be spread at all.

To meet all of the requirements for use in the practical. manufacture of the paper substitutes, the solvent must be capable of dissolving the isotactic polypropylene in concentrations of 10% to 80% (depending on the intrinsic viscosity of the polymer) to yield true solutions which can be readily spread on the support, and must be capable of being evaporated from the solution after spreading thereof, at a temperature below the melting point of the polyproylene (160 C.-l70 C.).

We have found that the petroleum fractions boiling between 100 C. and 250 C. meet all of the present requirements. Using those solvents, it is possible to prepare. solutions containing high concentrations of the isotactic polypropylene and which can be readily handled andspread. Thus, the concentration of the polypropylenefcan be as high as 80% for polymers having an intrinsic viscosity of about 0.6, and as high as 10% to 20% even when the intrinsic viscosity of the polypropylene is much higher and up to about 6.0.

- The isotactic polypropylene is practically insoluble in the petroleum fractions in the cold. Dissolution sets in at the elevated temperature (60 C.) and increases with increase in the temperature.

Figure 1 of the accompanying drawing shows the solubility curve for an isotactic polypropylene having an intrinsic viscosityof about 1. The concentrations of the isotactic polypropylene in the solution are reported on the abscissae, and the temperatures at which the curve was plotted are shown on the ordinates. Figure l was plotted for solutions of the polypropylene in n-decane but it was found that the solutions in the various petroleum fractions exhibited the same behaviour. I

As'is apparent from a study of the curve of Figure l, the temperature of dissolution of the isotactic polypropylene is about 96 C. for concentrations below 10% and increases to almost 160 C. for concentrations approaching 100%.

In practice, the isotactic polypropylene is dispersed in the solvent in the cold, the dispersion is heated to obtain a solution, the hot solution is spread in a thin layer on a fixed or mobile surface, and the solvent is evaporated at a temperature below 170 C., i.e. at a temperature of 100 C. to 160 C.

Surprisingly, the present sheets having a more or less rough surface identical with that of paper manufactured in thernormal way from fibrillating cellulosic fibers, are

obtained, rather than a smooth-surface film. This results from a phenomenon which occurs during the evaporation of the solvent from the thin layer of the solution, under the conditions of the" present method.

We find that, when a solution of the isotactic polypropylene in a petroleum fraction boiling between C. and 250 C. is spread in a thin layer on a support and the solvent is evaporated at 100 C. to C., that is at a temperature below the melting point of the polymer, a film is formed during the initial stages of the evaporation but that the curve of solubility is broken as the evaporation proceeds, and that as it passes from the solubility Zone into the insolubility zone, a portion of the polymer precipitates as a white, impalpable powder which becomes intimately associated with, or bound in, the layer of isotactic polypropylene. The final product is a dull white sheet comprising the layer of isotactic polypropylene having the portion of the isotactic polypropylene which precipitated during the evaporation of the solvent integrally and irremovably associated with it. The exposed surface of the sheet or pellicle (i.e. the surface from which evaporation of the solvent has taken place) is opaque and more or less rough in a way completely similar to the surface normally possessed by paper. .The opposite surface of the sheet (which remains in contact with the supporting surface during evaporation of the solvent) has, if the supporting surface is smooth, a smooth, lucid appearance. Under certain conditions as described further hereinbelow and with the use of opacifying agents such as Ti0 mixed with the polymer solution, or by spreading the solution on a surface that has been roughened by grinding with an emery wheel or the like, it is possible to obtain a sheet of polypropylene both surfaces of which are completely similar to the more or less rough surface of conventional paper.

The amount of the polypropylene which precipitates as the white impalpable powder during transition of the polymer from the soluble to the insoluble stage as the evaporation of the solvent proceeds, to yield the sheet having at least one surface identical to the surface of paper, can be controlled not only by controlling the temperature of the evaporation but also by varying the concentration of the polypropylene in the solution spread on the support. In general, the higher the concentration of the polymer in the solvent, the larger the amount of the polymer precipitated. The formation of the layer of precipitated white impalpable powder, which is always intimately associated With an integral with the sheet as a whole, is also favored, and can be controlled, by cooling the solution for some minutes after it is spread on the support and before it is heated to facilitate evaporation of the solvent. I

Figure 2 of the drawing is a graphic illustration of what occurs when a thin layer of a solution of the isotactic polypropylene is evaporated at a constant temperature T. In Figure 2, A represents the point at which precipitation of the polymer as the white impalpable powder begins, and point B represents the point at which the evaporation of the solvent is complete. By cooling the resulting sheet to point P (room temperature), a final sheet similar to paper is obtained.

The temperature at which the evaporation is eiiected is critical, since if the petroleum fraction is evaporated at a temperature above about 160 C., the sheet obtained is not similar to paper but is a more or less transparent, sniiooth film which cannot be printed or written on with in The new synthetic paper of this invention has many important advantages over conventional paper. Owing to the chemical constitution of the isotactic polypropylene, the sheet is hydrophobic and completely resistant to moisture and water. It retains its mechanical strength, which is inherently greater than that of paper, even when it is immersed in water for indefinite periods of time. These paper substitutes made from the isotactic polypropylene as described herein have a density lower than that of water and float on water when placed therein.

The new products of the invention are also entirely resistant to creasing, even on repeated bending. Another advantage of the synthetic paper of the invention over conventional paper made from cellulosic fibers is that the polypropylene from which the new sheets are formed melts at160 C.-170 C. without decomposition whereas cellulose decomposes and carbonizes at temperatures between 150 C. and 180 C.

These characteristics of the isotactic polypropylene render it particularly suitable for the manufacture of sheets having the surface characteristics of paper but adapted to use under conditions for which paper is not suitable, for example, the manufacture of sheets to be used as paper but under conditions of severe exposure to wear and/ or weathering.

For example, the paper-like sheets of the polypropylene are suitable for use in preparing nautical charts, geographical maps, papers for use by the military and, in general, for preparing documents, books, writing tablets, and permanent records of all kinds. The new sheets are also suitable for packing, for various household purposes, as substitutes for grease-resistant paper and as insulating papers for electric cables, etc.

The new synthetic paper comprising the isotactic polypropylene can be printed and written on easily and legibly with the different kinds of ink, for example with ink based on iron gallate or tannate, indelible inks, invisible inks, etc.

The following examples are given to illustrate the invention, it being understood that these examples are not intended as limitative.

EXAMPLE 1 A 45% solution of a polypropylene having an intrinsic viscosity of 3.2 is prepared by dispersing the polymer in a petroleum fraction boiling at 160-200 C. in the cold (room temperature) and then heating the dispersion to 160 C. The solution is spread on a perfectly smooth support which is placed in an oven at 145 C. for 15 minutes in order to evaporate the solvent. The film thus formed is quenched in water in order to improve its flexibility.

A white sheet is thus obtained having the appearance of paper. The mechanical characteristics of the sheet are determined by using the electronic dynamometer Instrom according to DIN specifications. For purposes of comparison, the same determinations are carried out on 3 types of conventional paper, that is, a block writing paper, magazine paper, and graph paper. The following results are obtained:

Ultimate Elonga- Shear Tear Kind of Paper Strength, tion, Strength, Strength, lrgJmm. percent kg./cm. kgjcm.

Polypropylene 2 24 32 1. 5 Writing block 1.87 3. 45 0. 2 0.78 Magazine paper 1. 3 3. 45 0.2 1. 8 Graph paper 4 3. 45 0. 4 2

The polypropylene paper can be written on with a pencil or with a pen, or printed with lithographic or printing inks. The writing was clear and resistant to rubbing.

EXAMPLE 2 an elongation of 20%, a shear strength of 32 kg./cm., and a tear strength of 1.7 kg/mm.

EXAMPLE 3 A sheet of synthetic paper was prepared as in the preceding examples, but using an solution of a polypropylene having an intrinsic viscosity of 0.8 in a petroleum fraction boiling at 170-190" C. The solution was heated to 165-170 C., spread as a thin layer on the support, slightly cooled in the air for 3 minutes, dried in an oven at 150 C. for 5 minutes, and finally quenched in cold water. The white sheet of synthetic paper obtained had an ultimate strength of 1.9 kg./mrn. elongation 25%, shear strength 31 kg./cm., tear strength 1.5 kg. mm.

EXAMPLE 4 A sheet of synthetic paper is prepared as in the preceding examples, but using a 25% solution of a polypropylene having an intrinsic viscosity of 1.3, in a petroleum fraction boiling at 190-210 C., and which is spread on the support as a thin layer, cooled in the air for 5 minutes, and dried in an oven at C. for 30 minutes.

A white sheet is obtained having the acteristics: ultimate strength 2.3 22%, shear strength 33 kg./cm., min.

The sheet obtained, which is similar to paper, has a white, dull side and a transparent, lucid side.

Many times it is desirable that writing on one side of paper be invisible on the opposite side. We have found that, by incorporating a white pigment in the solution of the polypropylene, e.g., titanium dioxide, in the proportion of 01-20% of the polymer, a nontransparent material, completely similar to paper is obtained, on one side of which it is possible to write with out the writing being visible from the opposite side.

The addition of the opacifying agent does not effect the manufacturing process.

EXAMPLE 5 A dispersion of 22.5 parts of polypropylene having an intrinsic viscosity of 2.4 and 2.5 parts of "H0 in 75 parts of a petroleum fraction boiling at 190 -210 C. was prepared in the cold. The dispersion was heated to C., spread as a thin layer on a metal laminae, and the solvent was evaporated at 150 C. for 20 minutes.

After quenching in water, an opaque sheet was obtained on which one could write, the writing being invisible from the opposite side of the sheet which had been in contact with the support.

The sheet had the following characteristics-ultimate strength 2.2 kg./mm. elongation 23%, shear strength 22 kg./cm., tear strength 1.6 kg./mm.

The sheet of synthetic paper obtained according to the preceding examples, especially the sheet obtained by evapcrating the solutions at a temperature close to C., has, as previously mentioned, a surface (the exposed surface from which the solvent is evaporated) which can be printed or Written on and a surface (the surface which, during the evaporation, was in contact with the support) which is rather smooth and lucid, on which it is difiicult to write and the writing can be partially removed by rubbing. By using a fiat support, for example glass or metal, previously ground with an emery consisting of grains having a diameter of 1-200 microns and preparing the sheets of paper as described in the preceding examples, sheets are obtained on both surfaces of which it is possible to write legibly. This is illustrated in Example 6. The sheets obtained have substantially the same mechanical characteristics as the sheets described in the preceding examples.

following charkgjmmfi, elongation tear strength 1.6 kg./

EXAMPLE 6 A dispersion of 22 parts of polypropylene having an intrinsic viscosity of 3.0 and 3 part of Ti in 75 parts of a petroleum fraction boiling at 170190 C. was prepared in the cold.

The dispersion was heated to 150 C., spread as a thin layer on a ground metal laminae and evaporated at 150 C. for 20 minutes. After quenching in water, a sheet was obtained on both sides of which it was possible to write easily and legibly.

The sheets had the following characteristics-ultimate strength 2.3 kg./mm. elongation 25%, shear strength 23 kg./crn., tear strength 1.7 kg./mm. I

It is also possible to apply a solution of the polypropylene in petroleum fraction boiling at 100250 C. as a thin layer directly on both sides of a transparent, stretched film of polypropylene, and evaporate the solvent at a temperature not higher than 160 C., to obtain a very thin layer of opaque polymer on both sides of the film, which layer imparts the appearance of a sheet of paper to the film. This is illustrated in Example 7.

EXAMPLE 7 A 20% solution of polypropylene having an intrinsic viscosity 2.3 in a petroleum fraction boiling at 170190 C. is spread as a thin layer at 150 C. on both sides of a film of polypropylene having an intrinsic viscosity 1.2, the film having a tenacity of 24 kg/mm. and an elongation of 37%.

On evaporation of the solvent at 140 C., a sheet is obtained on both sides of which it is easy to write.

The polypropylenes used in practicing this invention are normally either substantially crystallizable, linear, regular head-to-tail, high molecular weight polymers having an intrinsic viscosity above 0.5, or mixtures of such polymers with minor amounts (not over 30%) of amorphous, linear, regular head-to-tail polypropylene.

The intrinsic viscosities for the polypropylene reported in the examples were determined in tetralin at 135 C.

As previously mentioned and as disclosed in the pending applications supra, the polymer comprising the crystallizable polypropylene may be obtained by polymerizing propylene with the aid of a catalyst obtained from a transition metal compound and a metal alkyl, inan inert hydrocarbon solvent.

The transition metal compound consists of a compound or a mixture of compounds of a transition metal of groups IV to VI of the periodic table. It may be, for example, a halide of such transition metals as titanium, zirconium, hafnium, thorium, vanadium, tantalum, niobium, chromium, molybdenum, tungsten and uranium.

The metal alkyl compound comprises a substance or a mixture of substances selected from the group consisting of. simple and complex compounds the molecules of which contain an element from the group forming the 2nd and 3rd columns of the periodic table, i.e., beryllium, magnesium, zinc, cadmium, and other elements of the 2nd column, aswell as aluminum and other elements of the 3rd column. V

The valences of the element of the 2nd or 3rd column of the periodic table are linked to thesame or different alkyl radicals such as ethyl, propyl, butyl, etc.,-i.e., alkyl radicals of from 1 to 16 carbon atoms. One valence of said element may be satisfied by halogen or analkoxy radical of from 2 to 16 carbon atoms. Typical organometallic compounds include triethyl aluminum, diethylmono-chloro-aluminum, diethyl zinc, etc.

Inert solvents suitable for use in preparing the catalyst are paraifinic hydrocarbons such as a light gasoline substantially free of olefinic bonds, n-heptane, and isooctane. Anhydrous benzene may be used.

Asuitable molar ratio of the transition metal compound to the metal alkyl is 1:1 to 1:10, usually preferably 1:1 to 1:6.

The polymerizationofpropylene with the aid of these catalysts may be carried out at temperatures between 50 C; and 100 C., at normal atmospheric pressure or at somewhat increased pressure, e.g., at a pressure between normal atmospheric and 30 atmospheres.

The polymerization reaction mass comprises, as impurities, residual catalyst and inorganic compounds resulting from decomposition of the catalyst. The mass is therefore treated with a suitable agent such as methanol for decomposing the residual catalyst and then purified by the addition of methanol acidified with HCl.

The initial polymerization product is, usually, a mixture of polymers generally comprising a small amount of an oily low molecular weight fraction, an amorphous fraction of higher molecular weight, and a high molecular weight, crystallizable fraction. The oily low molecular weight polymers can be separated by extraction with acetone.

if the amount of amorphous polymer remaining after the acetone extraction is not over 30% by weight, the mixture of amorphous and crystallizable polymers may be used without further fractionation for the present purposes. However, if the amount of the amorphous polymers is greater than about 30%, such polymers, or a proportion thereof, are preferably removed from the polymerizate before the latter is used, by further extraction of the mass to leave a residue substantially consisting of crystallizable polypropylene or comprising a mixture of the crystallizable polypropylene and not more than 30% of the amorphous polymer. The residue of the acetone extraction may be extracted with ether or it may be extracted successively with ether and n-heptane.

When the catalyst is crystalline and insoluble or difficultly dispersible in the inert hydrocarbon, for instance when it is the reaction product of titanium trichloride and aluminum triethyl, the initial polymerizate may consist substantially to essentially of the cry'stallizable (isotactic) polypropylene.

The polymers have high molecular weights above 1000 and as high as 100,000 or even much higher.

The following examples are given to illustrate the production of specific polypropylenes which may be used in the practice of this invention.

Example A To a solution of 7.8 gms. of tripropyl aluminum in cc. of heptane are added, dropwise at 0 C. and under nitrogen, 1.9 gms. of titanium tetrachloride dissolved in 25 ccs. of heptanef The suspension is diluted to 200 ccs. with'heptane and introduced under nitrogen into a 435 cos. autoclave. After 102 gms. of propylene have been added, the autoclave is heated, with stirring of the contents, to 60 C. and is kept for about 40 hours at between 60 C. and about 68 C. The unreacted gasesare then vented and 5 0 ccs. of methanol are pumped into the autoclave.

'The coagulated polymerizate thus obtained is purified,

and 17.2 gms. of solid'polymeric material are obtained and fractionated by hot solvent extraction.

The acetone extracted fraction A (27.6% of the total polymer) consists of semi-solid products of low molecular weight.

The ether extracted fraction B (26.9% of the total) consists of a solid product of gummy appearance having an intrinsic viscosity of 1.57 corresponding to an average molecular weight of about 63,000. This fraction which comprises a mixture of polymers, is amorphous under the X-rays.

The heptane extracted fraction C, amounting to 15.1% of the total polymer, consists of partially crystalline polypropylene having an intrinsic viscosity of 2.35 corresponding to a molecular weight of approximately 120,000. The extraction residue is formed by highly crystalline polypropylene having an intrinsic viscosity of 5.1 (molecular weight approximately 390,000).

Example B Two steel balls, a glass vial containing 7.2 gms. of

crystalline titanium dichloride and a solution of 11.4 gms. of triethyl aluminum in 500 ccs. of n-heptane are introduced under nitrogen atmosphere into a 2150 cc. autoclave. The autoclave is then heated to 82 C. and at that temperature 140 gms. of pure propylene are introduced. The autoclave is then set in motion in order to break the vial. This leads to the formation of a coarsely dispersed solid polymerizing agent. The autoclave is kept in motion for about 10 hours at 80 to 85 C. Thereafter, the gases are vented and the unpolymerized propylene is collected.

After pumping methanol into the autoclave, the polymer is taken out as a white powder and is purified with acids to eliminate the inorganic products present. About 115 gms. of a white powder are obtained with conversion of 82% on the used propylene.

The polymer obtained is fractionated by extraction with hot solvents.

The oily, low molecular weight polymers, 5.8% of the obtained product, are removed by extraction with hot acetone. Extraction of the residue with hot ether dissolved an amount of polymer (8.3% of the total polymer) which consisted of solid polypropylene, amorphous under the X-rays, having an intrinsic viscosity of 0.47 in tetralin solution at 135 C.

By hot extraction with n-heptane a fraction was then obtained (corresponding to 10.4% of the total) consist ing of polypropylene having an intrinsic viscosity of 0.57 and more than 50% crystalline under the X-rays. The extraction residue, corresponding to 75% of the total polymer, consists of a highly crystalline polypropylene having an intrinsic viscosity of 1.86. The raw polymer obtained had, therefore, a crystallinity of at least 80%.

Some changes may be made in practicing this invention without departing from the spirit and scope thereof. It

1. As an article of manufacture, a sheet of polypropylene having an intrinsic viscosity of at least 0.5 as measured in tetralin at 135 C. and consisting for at least 70% of isotactic macromolecules non-extractable with boiling n-heptane, said sheet having at least one of its surfaces opaque and composed of precipitated polypropylene which is integral with the sheet and renders said surface rough in texture and ink-receptive.

2. As an article of manufacture, a sheet of polypropylene having an intrinsic viscosity of at least 0.5 as measured in tetralin at 135 C. and consisting for at least 70% of isotactic macromolecules non-extractable with boiling n-heptane, said sheet having One of its surfaces opaque and composed of precipitated polypropylene which 1s integral with the sheet and renders said surface rough in texture and ink-receptive, the other surface of the sheet being smooth, transparent and lucid, and printing or writing on the opaque surface being invisible from the transparent surface.

3. As an article of manufacture, a sheet of polypropylene having an intrinsic viscosity of at least 0.5 as measured in tetralin at 135 C. and consisting for at least 70% of isotactic macromolecules non-extractable with boiling n-heptane, said sheet having both of its surfaces opaque and composed of precipitated polypropylene which is integral with the sheet and renders said rough in texture and receptive to ink.

4. A process for making a sheet from a polypropylene having an intrinsic viscosity of at least 0.5 as measured in tetralin at 135 C. and consisting for at least 70% of isotactic macromolecules non-extractable with boiling n-heptane, which sheet has a surface composed of precipitated polypropylene which is integral with the sheet and renders the surface rough in texture and receptive to ink, said process comprising spreading a 10% to solution of the polypropylene in an organic solvent therefor as a thin layer on a support, and then evaporating the solvent at a temperature below the melting point of the polypropylene and below about 170 C.

5. The process according to claim 4, characterized in that the solution of the polypropylene is spread on a support having a surface roughened by grinding.

6. The process according to claim 4, characterized in that the surface on which the solution of the propylene is spread is a pre-formed, stretched and oriented film of the polypropylene.

7. The process according to claim 4, characterized in that the polypropylene made into the sheet has an intrinsic viscosity of 0.5 to 6.0, determined in tetralin at C.

8. The process according to claim 4, characterized in that the solvent is a petroleum fraction having a boiling point between 100 C. and 250 C., and the solvent is evaporated at a temperature between 100 C. and C.

9. As an article of manufacture, a paper-like sheet consisting essentially of a polypropylene having an intrinsic viscosity of at least 0.5 as measured in tetralin at 135 C., said polypropylene being comprised for at least 70% of isotactic macromolecules non-extractable with boiling n-heptane, said paper-like sheet having at least one of its surfaces opaque and composed of precipitated polypropylene which is integral with said sheet so that said surface is rough in texture and ink receptive.

10. As an article of manufacture, an ink-receptive body consisting essentially of a polypropylene having an intrinsic viscosity of at least 0.5 as measured in tetralin at 135 C., said polypropylene being comprised for at least 70% of isotactic macromolecules non-extractable with boiling n-heptane, said body having at least one of its surfaces opaque and composed of precipitated polypropylene which is integral with said body so that said surface is rough in texture and ink-receptive.

11. A process for making a body having an ink-receptive layer, said layer being comprised of a polypropylene having an intrinsic viscosity of at least 0.5 as measured in tetralin at 135 C., and consisting for at least 70% of isotactic macromolecules non-extractable with boiling n-heptane, which layer has a surface composed of precipitated polypropylene which is integral with the layer and renders the surface rough in texture and receptive to ink, said process comprising spreading a 10% to 80% solution of the polypropylene in an organic solvent therefor as a thin layer on the body, and then evaporating the solvent at a temperature below the melting point of the polypropylene and below about C.

surface References Cited in the file of this patent UNITED STATES PATENTS 

2. AS AN ARTICLE OF MANUFACTURE, A SHEET OF POLYPROPYLENE HAVING AN INTRINSIC VISCOSITY OF AT LEAST 0.5 AS MEASURED IN TETRALIN AT 135*C. AND CONSISTING FOR AT LEAST 70% OF ISOITATIC MACROMOLECULES NON-EXTRACTABLE WITH BOILING N-HEPTANE, SAID SHEET HAVING ONE OF ITS SURFACES OPAQUE AND COMPOSED OF PRECIPITATED POLYPROPYLENE WHICH IS INTEGRAL WITH THE SHEET AND RENDERS SAID SURFACE ROUGH IN TEXTURE AND INK-RECEPTIVE, THE OTHER SURFACE OF THE SHEET BEING SMOOTH, TRANSPARENT AND LUCID, AND PRINTING OR WRITING ON THE OPAQUE SURFACE BEING INVISIBLE FROM THE TRANSPARENT SURFACE. 